This invention relates generally to systems for asymmetric digital subscriber loop (ADSL) communications.
Modems (short for “modulator demodulator”) are used to transfer data between processor-based systems. Generally, modems may be utilized to transmit information between processor-based systems over telephone lines. A pair of modems are coupled through a transport such as a telephone network. Each modem includes a transmitter and a receiver which may be coupled by an elastic store or hybrid. In general, digital information developed by a processor-based system may be converted to analog information for transmission over the transport. Likewise, analog information received from the transport may be converted to digital information for use by the processor-based system. Thus, on each end of the transport, a modem may be provided.
A modem that is used with personal computers, as an example, may be called a remote modem because it is remote from the telephone network's central office. A modem that is provided by a telephone system is generally called a central office modem.
ADSL modems may use frequency division multiplexing (FDM) or echo cancellation (EC) to achieve full duplex operation over a subscriber loop. Discrete multi-tone (DMT) is a multi-carrier modulation technique that may achieve high bandwidth efficiency. A central office ADSL modem transmits a downstream signal to a modem at a remote terminal. The central office modem receives an upstream signal from the remote modem. The upstream and downstream signals use a common transport, typically a telephone line. The upstream signal may carry data on a lower portion of a band of frequencies. The downstream signal may carry data over an upper portion of a band of frequencies. In some embodiments, a wider bandwidth may be utilized for downstream signals than upstream signals.
Existing ADSL modems generally are implemented using two or more integrated circuits. One set of integrated circuits provides most of the digital signal processing and the other provides the analog-to-digital and digital-to-analog conversion. Generally, the two integrated circuits are separated after analog to digital conversion on the receiver side and before the digital-to-analog conversion on the transmitter side.
This means that data is transmitted between the two chips at a relatively high data rate. This high data rate transmission between integrated circuit chips results in more buffering at each chip and more pins are needed to connect the chips. This increases the cost of each chip. In addition, the high data rate also results in higher system cost due to the impact of higher frequency operation on electromagnetic interference (EMI) shielding and power control.
Thus, there is a continuing need for an ADSL modem that allows data to be more efficiently shared between integrated circuits.